Oriented strand board is a wood-based construction sheathing product comprised of wood strands that are sliced from logs, dried, mixed with relatively small quantities of wax and resin, typically less than 3.5% by total weight, formed in mats with orientation of the wood strands controlled in the length and width directions. The mats are then pressed under heat and pressure, and thermosetting polymeric bonds are created, binding together the adhesive and wood strands to achieve rigid, structural grade panels. It is during this pressing and consolidation process that the wood is compressed by a factor of 1.35 to 1.70 times its original density. The final panels are dry when made. When used in construction, they will often take on moisture from ambient air and/or precipitation, thus exerting swelling forces on the panel as it attempts to regain its natural form and density.
Wood panels, and more particularly OSB, are omnipresent in the building industry. In recent years, the market of OSB panels has significantly increased with the displacement of plywood panels in construction markets due to the fact that the structural performance of OSB can match that of plywood, at a lower cost. However, because of their composition and manufacturing process, OSB panels have one recognized disadvantage: the panels swell in thickness when they come in contact with water or moisture. In the initial wetting cycle, following manufacture, most of this swelling is irreversible. Plywood, also a manufactured wood panel, does not exhibit thickness swelling to the same extent as OSB. The major difference is that the wood elements used to make plywood, i.e., veneers, are not densified to any great extent in the manufacturing process, and therefore, compressive stress is minimized.
This characteristic differential in swelling properties is evident in building construction, where panels can be exposed to the elements during construction. OSB will tend to exhibit more evidence of thickness swell perpendicular to the panel face, including edge flaring, than plywood. This creates the impression of an inferior product, even though OSB's structural performance properties are still intact. A standard test in the wood panel industry to measure the completeness of cure of the wood/glue bonds is to place a sample in boiling water for 2 hours. This also creates extreme conditions for swelling of the wood elements in the panels. Experiments show that the thickness of an-OSB panel ranging from 1/4" to 3/4" tested under these conditions, although it remains sound with structural integrity intact, will increase by 40%-60%, compared to its original thickness, and that this swelling is substantially irreversible. Plywood, on the other hand, swells less than 10% in a similar situation.
The swelling characteristics of OSB can be altered during manufacture, with use of more resin binders, or longer press time. However, in every instance, this adds significantly to the cost, with marginal improvements.
U.S. Pat. No. 3,173,460 (Hann) discloses a particleboard exposed to steam at a pressure between 30 and 100 psi while the board is restrained for a period of 1 to 10 minutes at temperatures between 135-170.degree. C. to prevent swelling. The treated board is then dried to a moisture content of less than 4%.
U.S. Pat. No. 4,893,415 (Moldrup et at.) describes a method of removing moisture from wood and wood-based products, i.e., a drying process, by first removing air in the drying chamber and then injecting superheated steam. Layers of the wood or wood-based products are disposed in a chamber and the layers are separated by beams, thus leaving empty spaces therebetween.
U.S. Pat. No. 4,017,980 (Kleinguenther) discloses a process and apparatus for drying fibrous materials under controlled conditions. The chamber comprises means to apply mechanical pressure to the materials and also apertures or openings used for the injection of steam or for creating a vacuum. Sheets or panels of any dimensions can be placed between plates and a predetermined force is applied.
In U.S. Forest Service Research Note FPL-0187, March 1968, Heebink et al. teach the post-treatment of stacks of phenolic resin-bonded particleboard with steam. The authors state that a 10 minutes post-treatment with steam at a temperature between 150 and 180.degree. C. reduced swelling and springback of panels treated thereby. The steaming treatment is said to be more effective without restraint against increase in thickness than with restraint or over-restraint.
In view of the above, it is apparent that there is a great need to develop a novel method for making dimensionally stable wood panels, and more particularly OSB, that could sustain prolonged exposure to water or moisture without showing significant swelling and not affecting the structural and physical properties of the panels.